Fatigue Testing of Controlled Memory Wire Nickel-Titanium Rotary Instruments

Introduction: To improve the fracture resistance of nickel-titanium (NiTi) files, manufacturers have introduced new alloys to manufacture NiTi files and developed new manufacturing processes. This study was aimed to examine the fatigue behavior of NiTi instruments from a novel controlled memory NiTi wire (CM Wire). Methods: Instruments of Pro File, Typhoon (TYP), Typhoon CM (TYP CM), DS-SS0250425NEYY (NEYY), and DS-SS0250425NEYY CM (NEYY CM) (DS Dental, Johnson City, TN) all size 25/.04 were subjected to rotational bending at the curvature of 35 degrees and 45 degrees in air at the temperature of 23 degrees +/- 2 degrees C, and the number of revolutions to fracture (N-f) was recorded. The fracture surface of all fragments was examined by a scanning electron microscope. The crack-initiation sites, the percentage of dimple area to the whole fracture cross-section, and the surface strain amplitude (epsilon(a)) were noted. Results: The new alloy yielded an improvement of over three to eight times in N-f of CM files than that of conventional NiTi files (P < .05). The vast majority of CM instruments (50%-92%) showed multiple crack origins, whereas most instruments made from conventional NiTi wire (58%-100%) had one crack origin. The values of the fraction area occupied by the dimple region were significantly smaller on CM NiTi instruments compared with conventional NiTi instruments (P < .01). The square (NEYY CM) versus the triangular (TYP CM) configuration showed a significantly different lifetime on CM wire at both curvatures (P < .01). Conclusions: The material property had a substantial impact on fatigue lifetime. Instruments made from CM Wire had a significantly higher Nf and lower surface strain amplitude than the conventional NiTi wire files with identical design. (J Endod 2011;37:997-1001)